Like the thread title asks: what is the point of GM's notes?
They are mnemonics. Any setting has the potential to have the detail and richness of own world. That quite a handful for an individual to keep track of especially for something that enjoyed as a hobby.
And gets better, suppose one is interesting in creating a setting in general? For a novel, for a film, for a RPG campaign? How does one organize the details, to extent details are created.
The ideal is that when a question is asked, we just recall the answer on the spot and quickly move on. But that a lot to ask especially if one is starting out to create a setting. And in my experience the process to get to that point differs for everybody. Sometimes perfect recall is not even the ideal endpoint if the interest is not there.
Whether perfect recall is the goal or something short of it, a person has to come up with mnemonics as an aid to recall the details of a setting. Ideally that mnemonics should be structured with how one things about the setting, how one organizes information, and most important done in a way that fun and interesting as a hobby.
Unfortunately there are myriad ways of doing this with written GM Notes being only one of the possibilities. I personally organize things in list in chronological order or in a spatial relationship. If ask me about the Kingdom of Kaldor in Harn it is because I have a list of what in Harn in my head and Kaldor is one of the top level entries. I know a great deal about Harn, my own Majestic Wilderlands, Greyhawk, and selected other settings. But more important I remember where I can find the details. Because while I can recite some facts about Kaldor in Harn, I definitely know where on my shelf in which book, and which chapter I can find anything written about Kaldor.
When It comes to the Majestic Wilderlands, I have more of it in my head than just about any other setting. But even there there are limits. So as with Harn, I know where to go onto my computer or filing box to find the details of a character or locale.
I know the discussion is up to page 95 now. But I hope folks find my insight to the OP useful.